Shapeshifters are creatures (human, elves, dwarves, the race doesn't matter) capable of transforming themselves into other creatures. Sometimes a shapeshifter prefers transforming into fearsome animals, but even more often they shapeshift into common creatures for passing unnoticed: For example, in Loren the Amazon Princess Chambara transforms herself into a giant scorpion and then into a black cat.
Shapeshifting can be an innate capability, or achieved through magic, usually Dark Magic.
It's not uncommon for shapeshifters who were born with that skill to be unaware of it. Usually it's a particular event that triggers the transformation, and until then, many natural-born shapeshifters think to be normal creatures like the others.